The invention concerns an X-ray analysis system for controlling the humidity of a sample, a humidifying system to humidify gases for use in such X-ray systems, as well as a method for operating an X-ray analysis system to control the humidity of a sample.
In performing X-ray diffraction measurements under certain conditions and with certain samples it is often desirable to perform measurements at non-ambient relative humidity (rh). Samples of interest for study under changing relative humidity conditions include pharmaceuticals, minerals (clays), zeolites as well as various fine chemicals. Pharmaceuticals and chemical materials often undergo phase transitions that are either humidity or temperature driven and dependent. For example, certain zeolites activate or deactivate their docking sites in the presence of moisture to change their catalyst capability. This phenomenon is temperature dependent. Similarly, clay minerals swell and interact with water vapor in a temperature related fashion. Thus, materials of interest to such industries need to be investigated at elevated humidity and elevated temperature. In particular, certain physical properties of such materials change as a function of humidity. These physical properties include lattice spacing, crystal volume, weight, density, electrical conductivity or resistance, electrical capacitance, biological activity as well as their optical transparency. In addition to the substances mentioned above, such changes are particularly important for investigations of cosmetics, textiles, polymers, fertilizer, explosives, food, desiccants, and paper.
Apparatus capable of generating a humidified atmosphere typically rely on the use of gases to which a certain control degree of water (water vapor) has been added. The properties and performance of such devices are defined in terms of certain characteristic parameters. These include the saturation vapor pressure Ps corresponding to the maximum possible partial pressure of water vapor at a given air temperature. This parameter is temperature dependent and dictates the largest degree of water vapor that can be contained in a certain amount of air at a given temperature. Other parameters of critical interest to such measurements include the absolute humidity corresponding to the water vapor content. This parameter can be given as a mixing ratio in units of grams per kilogram and is related to the concentration of water vapor (e.g. in grams per cubic meter or as a percentage of a certain volume) wherein these volume and mass related quantities are temperature and pressure dependent. The dew point temperature Dp in degrees centigrade gives an additional absolute measurement of humidity. There is a direct relationship between the dew point temperature and the saturation vapor pressure. The dew point temperature is that temperature at which the saturation vapor pressure (or saturation concentration) of the water is reached. Therefore, an indication of the dew point temperature at atmosphere pressure is a unique indication of humidity. In many applications, the convenient figure of merit for designating humidity is the relative humidity hrel, in percent. This quantity is the ratio between the absolute moisture and the absolute moisture for saturation at a given temperature and pressure. Since the saturation vapor pressure is temperature dependent, the relative humidity is also very strongly dependent on temperature. The relative humidity increases with decreasing temperature and decreases with increasing temperature for a given absolute humidity. When the relative humidity is 100% the vapor saturation pressure and the dew point temperature have been reached. Therefore, with measurements of the relative humidity, the measuring sensor must be at the same temperature as the medium being measured. Temperature deviations of only one degree can result in serious measurement errors.
Special humidity chambers have been developed for carrying out such humidity dependent X-ray diffraction measurement of samples. The sample is introduced into the chamber and the chamber provided with appropriate windows for entrance and exit of X-rays. Flanges or feed elements are provided in the chamber for introduction of a humidified gas and heating devices are provided in close proximity to the sample to heat the sample to the desired temperature. In this manner, both the humidity of the environment of the sample within the specialized humidity chamber as well as the sample temperature can be controlled in a defined fashion (see for example Anton Paar® GmbH, THC Temperature Humidity Chamber, “Relative Humidity and Medium Temperature Attachment”, Graz, Austria, May 2000, Company Brochure and Product).
In order to prepare the humidified gas for introduction into such chambers a so-called membrane humidifier tube or nested tubes have been used. Such double-tubed instruments are for example provided by the company PERMA PURE® (MH and PH®—Serres Humidifiers, PERMA PURE® Incorporated, Company Brochure, Novembre 1998) and are based on the use of a special material referred to in the industry as Nafion®. For such membrane humidifiers, water introduced to the outer side of the membrane comes into contact with and is spontaneously bound to sulphonic acid groups located within the membrane. In consequence thereof additional sulphonic acid groups located further within the membrane have a higher relative affinity to water than the groups already containing water and the water is consequently transported through ion channels until the opposite side of the membrane is reached and the membrane wall is saturated. This transport procedure occurs rapidly in the direction of the partial water pressure gradient. If liquid water is present on one side of the membrane and a flowing dry gas on the other side, a so-called “permeation distillation” can occur, since only water molecules can be transported through the ion channels. “Contaminants” in the water cannot therefore be transported into the gas. As long as the temperature of the gaseous portion of the instrument does not fall below the dew point temperature only water vapor can be present in the gas. The water transport properties depend on the temperature and on the flow of the gas that is to be moistened as well as on the length (the surface area) of the membrane tube. For advantageous configurations, the water pressure saturation point can be approached at high temperatures.
Such permeation distillation membrane humidifiers typically have a double tube construction in which the gas is present in an inner tube and water in the outer tube or vice versa. This double walled structure facilitates the application of moisturized gases to samples under positive pressures as well as to samples under vacuum. The water can be stationary or caused to flow in either the inner or outer tube and the flow rate of the gas can be adjusted in order to provide a humidity dependence as a function of various parameters. The inner tube is fashioned from the membrane humidifier and the outer tube typically from a corrosion resistance material.
Such moisturizing systems for use in X-ray diffraction spectrometer comprising a double tubed membrane humidifier system in combination with a specialized temperature humidity chamber have the disadvantage of being relatively sensitive to small changes in the operation parameters while only allowing certain restricted ranges of humidity at associated temperatures. In addition to the limitations concerning the temperatures and humidities at which samples can be measured, such systems tend to be expensive and complicated to construct and to use.
In view of these deficiencies in the prior art, it is the object of the present invention to present a method and apparatus for humidifying gases, in particular for use in X-ray diffraction spectrometers with which broad ranges of temperatures and humidity can be achieved using straight forward, stable, and relatively inexpensive technical means.